Mention that Mariatta is sponsor-able on GitHub (#495)

motivations.rst

@@ -144,9 +144,11 @@ participating in the CPython core development process:
      sometimes `speaks <http://mariatta.ca/pages/talk-chronology.html#talk-chronology>`_
      at conferences.
    * Email address: mariatta@python.org
+   * `Sponsor Mariatta on GitHub <https://github.com/users/Mariatta/sponsorship>`_
    * `Patreon <https://www.patreon.com/Mariatta>`_
 
-   Support Mariatta by sending her a `happiness packet <https://www.happinesspackets.io/send/>`_,
+   Support Mariatta by `becoming a sponsor <https://github.com/users/Mariatta/sponsorship>`_,
+   sending her a `happiness packet <https://www.happinesspackets.io/send/>`_,
    or `paypal <https://www.paypal.me/mariatta>`_.
 
 .. topic:: R. David Murray (United States)